Don't thin it at all if you're applying it to small areas to fill pinholes,
scratches, cracks, etc. This type of application would typically be done
using a brush, toothpick, etc.
If you're applying it over a large area (probably as a primer), use the
1000 and thin it with lacquer thinner. Experienced modelers don't bother
with thinning ratios, they just learn how it should look when thinned
enough for their purpose; for airbrushing, the rule of thumb is "thin it to
the consistency of milk", i.e. until the appearance of a drop running down
the side of a jar is similar to that of milk.
The 500 can do that if they're not too deep. Apply a few drops with a
brush or toothpick and let it dry. It'll shrink, so you may need to repeat
a few times until it's completely filled. Then wet-sand it flush.